go/ast: ast.Print must not crash with unexported fields

Don't print unexported struct fields; their values are
not accessible via reflection.

Fixes #3898.

Also:
- added support for arrays
- print empty maps, arrays, slices, structs on one line
  for a denser output
- added respective test cases

R=r
CC=golang-dev
https://golang.org/cl/6454089

src/pkg/go/ast/print.go

@@ -34,7 +34,8 @@ func NotNilFilter(_ string, v reflect.Value) bool {
 //
 // A non-nil FieldFilter f may be provided to control the output:
 // struct fields for which f(fieldname, fieldvalue) is true are
-// are printed; all others are filtered from the output.
+// are printed; all others are filtered from the output. Unexported
+// struct fields are never printed.
 //
 func Fprint(w io.Writer, fset *token.FileSet, x interface{}, f FieldFilter) (err error) {
 	// setup printer
@@ -145,15 +146,18 @@ func (p *printer) print(x reflect.Value) {
 		p.print(x.Elem())
 
 	case reflect.Map:
-		p.printf("%s (len = %d) {\n", x.Type(), x.Len())
-		p.indent++
-		for _, key := range x.MapKeys() {
-			p.print(key)
-			p.printf(": ")
-			p.print(x.MapIndex(key))
+		p.printf("%s (len = %d) {", x.Type(), x.Len())
+		if x.Len() > 0 {
+			p.indent++
 			p.printf("\n")
+			for _, key := range x.MapKeys() {
+				p.print(key)
+				p.printf(": ")
+				p.print(x.MapIndex(key))
+				p.printf("\n")
+			}
+			p.indent--
 		}
-		p.indent--
 		p.printf("}")
 
 	case reflect.Ptr:
@@ -169,32 +173,57 @@ func (p *printer) print(x reflect.Value) {
 			p.print(x.Elem())
 		}
 
+	case reflect.Array:
+		p.printf("%s {", x.Type())
+		if x.Len() > 0 {
+			p.indent++
+			p.printf("\n")
+			for i, n := 0, x.Len(); i < n; i++ {
+				p.printf("%d: ", i)
+				p.print(x.Index(i))
+				p.printf("\n")
+			}
+			p.indent--
+		}
+		p.printf("}")
+
 	case reflect.Slice:
 		if s, ok := x.Interface().([]byte); ok {
 			p.printf("%#q", s)
 			return
 		}
-		p.printf("%s (len = %d) {\n", x.Type(), x.Len())
-		p.indent++
-		for i, n := 0, x.Len(); i < n; i++ {
-			p.printf("%d: ", i)
-			p.print(x.Index(i))
+		p.printf("%s (len = %d) {", x.Type(), x.Len())
+		if x.Len() > 0 {
+			p.indent++
 			p.printf("\n")
+			for i, n := 0, x.Len(); i < n; i++ {
+				p.printf("%d: ", i)
+				p.print(x.Index(i))
+				p.printf("\n")
+			}
+			p.indent--
 		}
-		p.indent--
 		p.printf("}")
 
 	case reflect.Struct:
-		p.printf("%s {\n", x.Type())
-		p.indent++
 		t := x.Type()
+		p.printf("%s {", t)
+		p.indent++
+		first := true
 		for i, n := 0, t.NumField(); i < n; i++ {
-			name := t.Field(i).Name
-			value := x.Field(i)
-			if p.filter == nil || p.filter(name, value) {
-				p.printf("%s: ", name)
-				p.print(value)
-				p.printf("\n")
+			// exclude non-exported fields because their
+			// values cannot be accessed via reflection
+			if name := t.Field(i).Name; IsExported(name) {
+				value := x.Field(i)
+				if p.filter == nil || p.filter(name, value) {
+					if first {
+						p.printf("\n")
+						first = false
+					}
+					p.printf("%s: ", name)
+					p.print(value)
+					p.printf("\n")
+				}
 			}
 		}
 		p.indent--

src/pkg/go/ast/print_test.go

@@ -23,6 +23,7 @@ var tests = []struct {
 	{"foobar", "0  \"foobar\""},
 
 	// maps
+	{map[Expr]string{}, `0  map[ast.Expr]string (len = 0) {}`},
 	{map[string]int{"a": 1},
 		`0  map[string]int (len = 1) {
 		1  .  "a": 1
@@ -31,7 +32,21 @@ var tests = []struct {
 	// pointers
 	{new(int), "0  *0"},
 
+	// arrays
+	{[0]int{}, `0  [0]int {}`},
+	{[3]int{1, 2, 3},
+		`0  [3]int {
+		1  .  0: 1
+		2  .  1: 2
+		3  .  2: 3
+		4  }`},
+	{[...]int{42},
+		`0  [1]int {
+		1  .  0: 42
+		2  }`},
+
 	// slices
+	{[]int{}, `0  []int (len = 0) {}`},
 	{[]int{1, 2, 3},
 		`0  []int (len = 3) {
 		1  .  0: 1
@@ -40,6 +55,12 @@ var tests = []struct {
 		4  }`},
 
 	// structs
+	{struct{}{}, `0  struct {} {}`},
+	{struct{ x int }{007}, `0  struct { x int } {}`},
+	{struct{ X, y int }{42, 991},
+		`0  struct { X int; y int } {
+		1  .  X: 42
+		2  }`},
 	{struct{ X, Y int }{42, 991},
 		`0  struct { X int; Y int } {
 		1  .  X: 42